The signal loss due to susceptibility artifacts in T2*-weighted magnetic-resonance tomography images is a disadvantage of gradient-echo-based sequences such as, for example, FLASH, echo-planar imaging (EPI) and susceptibility weighted imaging (SWI). This problem is acute with radio-frequency MRT, but also with standard field strengths for clinical devices of, for example, 1.5 T. This signal loss occurs due to the fact that introducing the patient into the main magnetic field of a magnetic resonance system disrupts the homogeneity of the main magnetic field because different body tissues have different susceptibilities and therefore result in distortions of the magnetic field. This effect is particularly pronounced at severe susceptibility changes (e.g., at the transition between air and tissue), such as that which occurs, for example, at the frontal and paranasal sinuses in the head. These susceptibility magnetic field gradients, and hence gradients in the main magnetic field, dephase the Spins more quickly (e.g., T2* is shorter) and this dephasing may not be reversed by a gradient echo. Therefore, in the human brain, significant signal losses mainly occur in the frontal-orbital and inferior-temporal cortices and may significantly complicate the diagnosis of, for example, stroke and cerebral hemorrhage. In the case of a typical alignment of the slice images through the brain (e.g., parallel to the underside of the bar on a sagittal image), which corresponds approximately to an axial alignment, the signal void is particularly dominated by the component of the magnetic field gradient in the direction of the slice thickness (e.g., “through-plane component”). This negative effect is further intensified with lengthy echo times, such as are, for example, required for blood oxygen level-dependent (BOLD) functional magnetic resonance images (fMRIs).
Different approaches were suggested in the past to resolve this problem. For example, z-shim methods, with which z-gradients of different strengths are switched after the excitation pulse, exist. However, this uses numerous sub-images and hence extends the recording time. Additional B0 shim coils or diamagnetic shim materials may reduce the local inhomogeneity of the main magnetic field and hence the signal losses. However, this may requires manual intervention and is uncomfortable for the patient.
In the article by Weiran Deng, “Simultaneous Z-Shim-Method for Reducing Susceptibility Artifacts With Multiple Transmitters,” Magnetic Resonance in Medicine 61:255-259 (2009), it was suggested that a plurality of RF transmit coils be used. The slice-selective excitation pulse on the different RF transmit coils is to be sent with a different time shift in each case. This should result in a phase gradient that compensates the gradient in the main magnetic field in the direction of the slice thickness (e.g., with an exactly axial alignment of the slices in the Z direction). This is based on the knowledge that, with a slice-selective gradient Gz, a time shift or time delay τc of the excitation pulse with the RF pulse profile bc(t), where c designates the respective RF transmit coil, results in a linear phase shift along the slice profile m(z) (e.g., in the direction of the slice thickness). This is based on the formalism of the Fourier transformation, where a shift in the time domain generates a linear phase in the frequency domain. This is also known as the z-shim method. Deng suggests a simultaneous z-shim method for multiple RF transmit coils. The RF transmit pulses are sent with different time delays on separate RF transmit coils. The summation of the z-shims occurs automatically due to the parallel transmission. If the different RF transmit coils are sensitive in different ranges, this may also enable the magnetic field inhomogeneities that vary spatially over the area under examination to be compensated. Deng et al. demonstrated this with a local 4-channel head coil. However, this revealed strong B1 inhomogeneity effects that impair the image. No methods are suggested as to how the time delays may be calculated. Instead, the time delays were set manually. This makes this method unsuitable for clinical practice.